Semiconductor devices are becoming smaller and more dense with the evolution of new technology. However, increases in circuit density produce a corresponding increase in overall device enhancements in order to remain competitive. Semiconductor manufacturers are therefore constantly challenged to improve the quality, design and other aspects of their products. Whereas significant improvements have been made, however, these improvements alone are not sufficient in meeting all the challenges facing the semiconductor industry.
Assembly processes for electronic circuit devices use solder connections for electrically joining a semiconductor device to a support substrate and for device-substrate package interconnection to organic board. C4 (controlled collapse chip connection) technology, also called flip-chip bonding, is used to attach semiconductor chip to substrate. This involves connecting an array of solder bumps on the chip circuits structure side to module substrate bonding pads by heating the assembly to solder reflow temperature in the presence of solder flux to form a solder bond.
In multilayer ceramic (MLC) products, solder bumps on silicon device are generally of 97Pb/3Sn alloy, i.e., (97%Pb/3%Sn) which are deposited by standard evaporation technique.
However, the chip joining process involves application of a high temperature flux, typically rosin based Alpha-102 on the C4s' and/or on the solder wettable pads on the substrate. The chip having the C4 are aligned to the substrate bonding pads which is further facilitated by the flux viscosity and tackiness. The chip-substrate assembly is then subjected to solder reflow in a furnace under nitrogen or forming gas (5 percent Hydrogen with 95 percent Nitrogen) using a temperature profile with about 350 to about 365.degree. C. peak temperature.
Flip-chip attachment to multi-layer ceramic carrier using solder bumps normally comprise 95%Pb/5%Sn or 97%Pb/3%Sn alloy on the device circuit side are disclosed in U.S. Pat. No. 3,401,126 (Miller) and U.S. Pat. No. 3,429,040 (Miller).
Rosin flux is also used for solder connections using 90Pb/10Sn and eutectic solder (37%Pb/63%Sn) in the fabrication of ball grid arrays (BGAs), ceramic ball grid arrays (CBGA), ceramic column grid arrays (CCGA), SMT discretes, and seal band attachment to provide surface wettability of the contacting surfaces during solder reflow which is essential for solder bond integrity.
In the cooling cycle of the thermal profile for joining, the solder hardens and at the same time the residual flux vapors deposit on the various exposed surfaces. In subsequent step, the electronic assembly is subjected to solvent cleaning operation prior to further processing.
Under the high temperature solder reflow environment, the rosin-flux organics are mostly removed by thermal decomposition to volatile species but a small fraction of these thermally activated species undergoes crosslinking reactions resulting in resinous/carbonaceous by-products as residue on the C4 connections and all other surfaces on the device and the substrate side that are exposed to the volatile species during the solder reflow processing. The flux residue must be removed from all critical surfaces prior to further operation as otherwise it can lead to function failure during long term use due to stress corrosion during exposure to temperature and humidity environment. Therefore, it is necessary that after chip joining and other soldering processes during fabrication of electronic circuit assemblies, the flux residue be cleaned-off before subsequent operations.
Further need for removal of flux residue is dictated by the observation that if any residual film of flux residue remains on the substrate or device surface materials, it causes detriment to the adhesion of C4 epoxy encapsulant or underfill which is required for enhanced C4 fatigue life and C4 reliability during product on-off cycles. After encapsulation, the module assembly is subjected to burn-in and test.
Rosin fluxes are natural products comprising a complex mixture of cyclic hydrocarbon acids and the corresponding esters, alcohols, and decarboxylated products. Among the resin acids which are the major components of the mixture, abietic acid up to between about 50 to about 60 percent, hydroabietic acid, and dehydroabietic acid are the predominant components. These rosin fluxes are known to promote wetting of metal surfaces by their chemically reacting with oxide layer on the surface of tin and/or lead providing oxide-free metal exposed surface of high surface energy which thermodynamically should readily wet clean contacting metal surfaces on the substrate and provide reliable chip-to-substrate interconnection.
However, thermally activated flux species formed during solder reflow also protect the molten solder surface from reoxidation during the reflow process. In this process, abietic acid component of the flux is considered to react with tin or lead to form the corresponding abietate in which metal is bound to organic acid and redeposited on the substrate on cooling.
Rosin-based flux residue cleaning processes traditionally have employed halogenated hydrocarbons, such as, perchloroethylene, 1,1,1-trichloroethane, fluorochlorocarbons CFC-113, and CFC-112, and aromatic hydrocarbons as xylene. The halogenated solvents, however, are undesirable due to their harmful effects on the environment and on human health. Because of the hazards associated with these solvents, particularly perchloroethylene and trichloroethane, their use in industrial processes has become highly restricted in recent years. Specifically, this category of solvents have been identified as Hazardous Air Pollutants or HAP solvents, which are on the OSHA list of Suspected Carcinogens (cancer causing agents), and these are among the SARA Title-III reportable (Superfund Amendment & Re-Authorization Act) compounds that are on the TRI (Toxic Release Inventory) chemicals list.
In addition to these environmental and health hazards, 1,1,1-Trichloroethane like CFCs is also an Ozone Depleting Substance (ODS) that cause stratospheric Ozone depletion and thus it has been phased out while 1,1,2-trichloroethylene has been targeted for phase out by the year 2002. The CFCs are Ozone Depleting Substances use of which is banned in industrial applications. In the case of aromatic hydrocarbons as xylene, there are regulatory issues with its industrial use because it is among the HAP solvents and thus is subject to SARA title-III reporting requirements in addition to its being a VOC (Volatile Organic Compound). VOCs can enter into photochemical reactions with oxides of nitrogen (NOx) in the environment and produce smog which causes health problems. There are also safety concerns with xylene as it is a highly flammable and volatile solvent having flash point 85.degree. F. which requires special high cost equipment for chemical safety in manufacturing environment as well as for compliance with rules for air emissions of hazardous air pollutants.
Because of the environmental concerns and the health hazards associated in the use of halogenated solvents and aromatic hydrocarbons, such as, xylene, in industrial cleaning applications in general, there is currently a major focus on identifying environmentally safe replacements.
Various chemical suppliers and cleaning equipment manufacturers have made available several alternate organic solvents that are relatively safe and mostly exempt from environmental regulations, as well as water-based cleaning solutions and the necessary equipment for alternate organic solvent and water-based cleaning.
U.S. Pat. No. 5,340,407 (Bolden) describes a process of removing soldering flux and/or adhesive tape residue from a substrate. Bolden basically uses terpene-based cleaning compositions for flux residue removal from the surface of a printed circuit board, and also for the removal of the adhesive tape residue.
U.S. Pat. No. 4,276,186 (Bakos) describes a cleaning composition and use thereof. Bakos essentially uses a cleaning composition which includes N-methyl-2-pyrrolidone and an alkanolamine for flux residue cleaning from the surface of printed circuit boards.
Terpene-based microemulsion cleaning solutions such as the ones disclosed in U.S. Pat. No. 5,401,325 (Mihelic), for flux residue cleaning are based on mixture of surfactants, a water soluble glycol ether, a sparingly water-soluble organic solvent, and morpholine in water. These solutions are claimed to be effective in removing baked-on oil and carbon deposits, and oil grease from various electronic device surfaces.
U.S. Pat. No. 5,431,739 (Bengston) discloses a process for cleaning and defluxing parts, specifically electronic circuit assemblies. Bengston provides environmentally safe flux removing compositions, and basically uses aryl alcohols as benzyl alcohol in water as a cleaning medium for solder flux residue from mildly activated rosin flux (RMA), oils and other contaminants from the surface of circuit boards.
U.S. Pat. No. 5,395,548 (Pfahl) and U.S. Pat. No. 5,482,563 (Pfahl) are concerned with defluxing electrical assembles using non-azeotropic solvent compositions based on combination of fluorinated alcohol and terpene and/or n-methyl-pyrrolidinone (NMP) or a combination of non-halogenated alcohol and terpene.
U.S. Pat. No. 5,112,517 (Buchwald) is concerned with using halogenated hydrocarbons, such as, dichlorodifluoroethanes in conjunction with alkanols for removing resin fluxes and flux residues from printed circuit boards.
None of the references given above are concerned with removal of flux residue formed under-the-chip when rosin flux as Alpha-102 is employed in high temperature solder reflow process for joining a device chip to ceramic-chip-carrier according to C4 technology nor the removal of flux residue from soldering processes in BGA (ball grid array), CBGA (ceramic ball grid arrays, CCGA (ceramic column grid array) attachment to substrate. Moreover, the cleaning compositions employed for the purposes in these patents are based on aqueous alkaline solutions or organic solvents which are not considered suitable for the purpose of this invention due to concern for C4 corrosion and environmental hazard issues with some of the solvents described.
This invention however provides an environmentally safe and effective method for cleaning of rosin-based Alpha-102 flux residue that is formed on the device chip and the substrate surface during solder interconnections by high temperature solder reflow for chip attachment according to Controlled Collapse Chip Connections (C4) technology in multi-layer ceramic products, and for flux residue cleaning after solder joining processes for electronic package to substrate interconnections. In C4 joining of devices having quartz or polyimide terminal passivation layer, the critical surfaces that are exposed to flux residue contamination during reflow are: Pb/Sn solder connections; bonding metallurgy on the substrate side; substrate ceramic; and polyimide passivation layer on the device chip. Standard processes for under-the-chip flux residue removal subsequent to chip joining are based on perchloroethylene or aromatic hydrocarbons, typically xylene, both of which are hazardous solvents. The inventors have discovered a group of non-hazardous solvents for effective removal of rosin flux residue from various critical surfaces effected during high temperature solder reflow interconnection processes to provide superior replacement of environmentally hazardous perchloroethylene, 1,1,1-trichloroethane, xylene and related halogenated and aromatic hydrocarbon solvents.
The flux residue cleaning solvents according to this invention are characterized by the presence of two or more functional groups of varying polarity, specifically, --OH and --COOR; --OR and --COOR; --OR and OCOR; and --OH and --OR, where R is a hydrocarbon group C.sub.n H.sub.2n+1 with n=1-4. Representative solvents with these features are: lactate esters as ethyl lactate, butyl lactate; alkoxyesters as ethyl-3-ethoxypropionate; alkoxyacetates as propylene glycol methylether acetate, propylene glycol methylether propionate (Methotate), propylene glycol butylether, dipropylene glycol methylether, dipropylene glycol methylether acetate.
Unlike halogenated hydrocarbon solvents and aromatic hydrocarbons, the replacement solvents according to this invention have no environmental regulatory issues, have no problem of toxic air emissions or health hazards, and are generally Class II or Class III combustibles (flash point greater than about 100.degree. F.) which offers a significant advantage in terms of equipment requirement over xylene which is highly flammable.